The present invention relates to a runflat tire, more particularly to an improved sidewall structure being capable of reducing the tire weight.
As an old-type runflat tire, a pneumatic tire whose sidewall portion is reinforced with a crescent-shaped thick hard rubber layer disposed axially inside the carcass is well known in the art. In case of passenger car tire sizes, such a rubber layer requires a thickness of more than 15 mm through a wide range between the tread shoulder portion and the bead portion in order to fully support tire loads when the tire is deflated. This greatly increases the tire weight, and accordingly, dynamic performance under normal running conditions deteriorates.
As a technique to increase the load supporting power of the sidewall portion, such a structure that a plurality of relatively thin crescent-shaped rubber layers are sandwiched between the carcass plies has been disclosed in for example the laid-open Japanese patent application No. 10-71807 (corresponding to U.S. Pat. No. 5,795,416).
However, it is difficult for even this structure to satisfy the recent severe requirements for tire weight reduction based on the movement towards fuel economy, resources saving and the like.
It is therefore, an object of the present invention to provide a pneumatic tire, in which the load supporting power of the sidewall portion can be greatly improved by a combination use of reinforcing rubber layers and steel cord reinforcing layer, and thereby the tire weight can be reduced while maintaining or improving the runflat performance.
According to the present invention, a pneumatic tire comprises
a tread portion,
a pair of sidewall portions,
a pair of bead portions each with a bead core and a bead apex therein,
the bead apex made of rubber extending radially outwards from the bead core,
a carcass extending between the bead portions and composed of an inside carcass ply and an outside carcass ply,
the inside carcass ply extending between the bead portions through the tread portion and sidewall portions and turned up around the bead core in each bead portion from the inside to the outside of the tire so as to form a pair of turned up portions and a main portion therebetween,
the outside carcass ply disposed outside the inside carcass ply and extending to a position between the bead apex and the turned up portion in each bead portion,
each bead portion provided with a steel cord reinforcing layer comprising an axially inner portion and an axially outer portion which extend radially outwardly from the underside of the bead core along the axially inside and outside of the bead core, respectively,
each sidewall portion provided with an axially inner reinforcing rubber layer and an axially outer reinforcing rubber layer,
the axially inner reinforcing rubber layer disposed along the axially inner surface of the main portion of the inside carcass ply,
the axially outer reinforcing rubber layer disposed between the outside carcass ply and the axially outer portion.